1. Field of the Invention
The present invention relates to a valve structure of a hydraulic shock absorber for a vehicle.
2. Description of the Related Art
There has been known a hydraulic shock absorber which is interposed between a vehicle body and an axle in a vehicle, and cushions an impact applied to the vehicle from a road surface.
A standard valve apparatus of the conventional hydraulic shock absorber is structured, as shown in FIG. 3B, such that a first leaf valve 2 is provided in an opening portion of a flow passage 1A arranged in a partition wall member 1 constituted by a piston sectioning an oil chamber. A small-diameter leaf valve 3 (an intermediate sheet) is provided on an opposite side of the first leaf valve 2 to a side facing the partition wall member 1. A plurality of second leaf valves 4 are provided in an opposite side of the small-diameter leaf valve 3 to the side facing the first leaf valve 2. The leaf valves 2, 3 and 4 are laminated on the partition wall member 1 by fixing inner peripheral sides of the respective leaf valves 2, 3 and 4 by a washer 5 and a nut 6. An annular gap is provided in an outer peripheral side of the small-diameter leaf valve 3, between the first leaf valve 2 and the second leaf valve 4. A damping force property B obtained by the standard valve structure in FIG. 3B, causes the changes of an expansion side damping force Ft with respect to a piston speed Vp and an expansion side damping force Fc being linear, as shown in FIG. 4. Accordingly, a soft ride quality can be obtained by reducing damping force in a low speed range. However, the damping force in a middle speed range can not be made appropriately high, and vehicle body pitching control is not good in the middle speed range.
A preset valve structure of the hydraulic shock absorber described in Japanese Utility Model Application Laid-Open No. 4-97133 (reference document 1) is structured, as shown in FIG. 3C, such that the number of the leaf valve 2 in the standard valve structure is set to a plural number. An inner leaf valve 7 is provided between a plurality of first leaf valves 2, and an annular outer leaf valve 8 having a larger thickness than that of the inner leaf valve 7 is provided in an outer peripheral side of the inner leaf valve 7. In accordance with the preset valve structure in FIG. 3C, the first leaf valve 2 on a side of the small-diameter leaf valve 3 is previously deflected at the amount that the outer leaf valve 8 is thicker than the inner leaf valve 7. A preset load in accordance with the previous deflection is applied to the first leaf valve 2 on the side of the partition wall member 1. A damping force property C having a high damping force is exhibited as shown in FIG. 4. It is possible to set the damping force in the middle speed range appropriately high, and it is possible to improve the vehicle body pitching control in the middle speed range.
In accordance with the preset valve structure of the hydraulic shock absorber described in the reference document 1, the preset load generated by the deflection of the first leaf valve 2 on the side opposite to the piston is applied to the first leaf valve 2 on the side of the piston (on the side of the partition wall member 1) from an initial stage of the low speed range. The damping force in the low speed range becomes excessive, thereby deteriorating ride quality.